Intersubband semiconductor lasers (ISLs) are of great interest for mid-infrared (2-20 μm) device applications. They are the preferred optical source in two important windows of atmospheric transparency, namely those at 3-5 μm and 8-13 μm (See Capasso et al., Optics & Photonics News, V. 10, No. 10, pp. 32-37, 1999). Applications range from pollution detection and industrial process monitoring to military countermeasures.
As compared to interband midinfrared (lead salt) lasers, the unipolar ISL have the advantage of a higher temperature operation. An important drawback of contemporary ISL is low radiation efficiency resulting from high nonradiative intersubband electron relaxation in active quantum wells of ISL.
A number of techniques have been devised to enhance the lasing efficiency. The electron-recycling cascading scheme, in which electrons sequentially traversing N≈25 stacked active regions emit N laser photons, has been successfully used in quantum cascade (QC) ISL design (see Faist et al., Science, V. 264, No. 5158, pp. 553-556, 1994). A Bragg reflector region was incorporated in the QC ISL active region to provide strong electron confinement in the upper lasing state (see Faist et al., Applied Physics Letters, V. 66, No. 5, pp. 538-540, 1995). Plasmon-enhanced waveguide has been specially redesigned for ISL to maintain a high optical confinement and reduce optical losses for wavelengths in the second atmospheric window (Sirtori, 1995). An improvement of the device characteristics has been also achieved by special design of the injector/relaxation region, to provide for resonant carrier injection at laser threshold (see Gmachl et al., Applied Physics Letters, V. 72, No. 24, pp. 3130-3132, 1998).
One of the ways of enhancing the lasing efficiency is to increase the optical gain in the ISL active region. The existence of optical gain is the basis for the lasing action. In the case of the ISL, the optical gain results from the inverted electronic population between the quantum subbands in the active quantum wells of the laser heterostructure. Population inversion between the lasing subbands occurs when the electron lifetime in the lower subband (the depopulation time) is shorter than the time of the intersuubband relaxation from the higher subband. In A3B5-based heterostructures the intersubband relaxation is mainly associated with the relatively fast process of LO-phonon emission. This leads to the requirement of even faster depopulation of the lower lasing subband. A number of different approaches to this problem have been proposed after the pioneering work by Kazarinov and Suris (see Kazarinov et al., Soviet Physics-semiconductors, V.5, No. 10, pp. 707-709, 1971), where the intrawell intersubband relaxation was assumed as a depopulation mechanism for the lower lasing subband, as shown in FIG. 1, but no special relaxation process had been specified. Helm proposed using sequential resonant tunneling in multiple-quantum-well heterotructures, as illustrated in FIG. 2, as means for both populating the upper laser level and depopulating the lower one (see Helm et al., Physical Review Letters, V 63, No. 1 pp. 74-77, 1989). These structures, however, encounter serious difficulties related to charge accumulation and coordinate-dependent energy misalignment of the levels in the adjacent quantum wells (QWs). An alternative depopulation design involving resonant tunneling through an adjacent narrow quantum well, as illustrated in FIG. 3, was proposed by Borenstain and Kastalsky (see Borenstain et al., Applied Physics Letters, V. 55, No. 7, pp. 654-656, 1939; and Kastalsky et al., Applied Physics Letters, V. 59, No. 21, pp. 2636-2638, 1991). The role of resonant tunneling in this design was to perform electronic energy filtering, whereas the active double QW was separated from another pair of QWs by an ohmic n+“reservoir.” The resonant tunneling depopulation mechanism employs a resonant penetration of the wavefunctions of the final electron states. However, schemes involving resonant tunneling have a disadvantage of being very sensitive to layer width fluctuation and charge build up, so they have never been implemented in the QC laser design.
Interwell scattering assisted by LO-phonons has been considered as an effective mechanism for the lower subband depopulation in ISL (see Sun et al., IEEE Journal of Quantum Electronics, V. 29, No. 4, pp. 1104-1111, 1993). This mechanism was employed in the first successful implementation of the quantum cascade ISL at Bell Laboratories (see Faist et al., 1994, above). The Bell Labs group placed a “drain” level in an adjacent quantum well exactly one LO-phonon energy below the depopulated lower laser level in the active quantum well, as illustrated in FIG. 4. This design maximizes the interwell scattering mediated by long-wavelength optical phonons. Nevertheless, even in this successful design the typical depopulation time is of the same order of magnitude as the time of nonradiative intersubband relaxation. As a result, the low depopulation rate still remains a barrier for improving the ISL performance and search for a suitable solution continues in many laboratories.
Other suggested methods of enhancing the depopulation rate in ISL include interband electron transitions and interband tunneling through broken-gap heterointerface. The method of interband stimulated emission depopulation suggested by Kastalsky (see Kastalsky et al., IEEE Journal of Quantum Physics, V. 29, No. 4, pp. 1112-1115, 1993) and illustrated in FIG. 5, is based on the incorporation of an interband laser in the same design together with the intersubband laser heterostructure, i.e. two simultaneously operating laser mechanisms in the same heterostructure. The broken-gap heterostructure, suggested by Yang (Yang et al., Applied Physics Letters, V. 59, No. 2, pp. 181-182, 1991) and illustrated in FIG. 6, requires a special type of heterostructure material—InAs/GaSb type-II interface. Mid-infrared interband cascade laser based on type-II heterostructures is a rapidly growing new class of semiconductor IR light sources (see Yang, Microelectronics Journal, V. 30, No. 10, ppl 1043-1056, 1999).
Still another known technique involves the use of a superlattice active region, as suggested long ago by Yuh and implemented recently in the QC laser design by Scamarcio (see Yuh et al., Applied Physics Letters, V. 51, No. 18, pp. 1404-1406, 1987; and Scamarcio et al., Science, V. 276, No. 5313, pp. 773-776, 1997). In this technique, illustrated in FIG. 7, the lasing action takes place between the electron states at the edge of the first minigap. Owing to the high oscillator strength inherent to the zone-boundary optical transition and the fast intra-miniband electron relaxation and transport, one obtains an efficient depopulation of the uppermost electronic states in the lower miniband. Despite the large transition matrix element and the efficient population inversion, the inter-miniband emission spectrum is considerably broader than the intersubband spectrum of ISL, and hence elaborate heterostructure design is required to achieve high-performance lasing (see Tredicucci et al., Applied Physics Letters, V. 73, No. 15, pp. 2101-2103, 1998; and Tredicucci et al., Applied Physics Letters, V. 74, No. 5, pp. 638-640, 1999).
Recently, Harrison suggested to use the LO-phonon assisted scattering into a strongly couple subband of a double quantum well (see Harrison, Semiconductor Science and Technology, V. 12, No. 11, pp. 1487-1490, 1997). This method of lower level depopulation in the ISL employs the enhanced wavefunction overlap between electron states in energy-aligned subbands (proximity depopulation) and is illustrated in FIG. 8. In order to obtain the energy alignment, Harrison suggested using an asymmetric double quantum well heterostructure with different well material compositions. The lower laser level in the active QW and the drain level in the adjacent QW are designed to be close to the anticrossing condition and further tuned by the application of an external electric field. Proximity depopulation does not affect directly the electron states near the bottom of the depopulated subband, which are involved into a lasing transition. Only LO-phonon emission from the thermally excited high-energy tail of the electron distribution in the depopulated subband participates in the proximity-enhanced interwell transitions. This feature both limits the depopulation rate and introduces a strong temperature dependence of the depopulation process. Hu proposed to use a coherent control of the LO-phonon emission rate by an external microwave field to improve the inverse population ration in double-QW ISL heterostructures with anticrossing lower energy levels (see Hu et al., Applied Physics Letters, V. 73, No. 7, pp. 876-878, 1998). This method is of limited practical use since it requires an external source of microwave radiation.